"Our new projections are probabilistic, and we find that there will probably be between 9.6 and 12.3 billion people in 2100," says Prof. Raftery.
The potentially catastrophic consequences of an exponentially growing global population is a favorite subject for writers of dystopian fiction.
The most recent example, Utopia - a forthcoming David Fincher-directed series for HBO - won critical acclaim in its original incarnation on UK television for its depiction of a conspiracy-laden modern world where the real threat to public health is not Ebola or other headline-friendly communicable viruses, but overpopulation.
Fears over the ever-expanding number of human bodies on our planet are not new and have been debated by researchers and policy makers for decades, if not centuries. However, recent research by University of Washington demographer Prof. Adrian Raftery - using modern statistical modeling and the latest data on population, fertility and mortality - has found that previous projections on population growth may have been conservative.
"Our new projections are probabilistic, and we find that there will probably be between 9.6 and 12.3 billion people in 2100," Prof. Raftery told Medical News Today. "This projection is based on a statistical model that uses all available past data on fertility and mortality from all countries in a systematic way, unlike previous projections that were based on expert assumptions."
Prof. Raftery's figure places up to an additional 5 billion people more on the Earth by 2100 than have been previously calculated.
A key finding of the study is that the fertility rate in Africa is declining much more slowly than has been previously estimated, which Prof. Raftery tells us "has major long-term implications for population."
Fertility rates declining more slowly in Africa than previously reported
A 2003 Centers for Disease Control and Prevention (CDC) report found that, in sub-Saharan Africa, both fertility and mortality rates were high, with the proportion of people aged over 65 expected to remain small, increasing from an estimated 2.9% in 2000 to 3.7% in 2030.
The CDC report notes that fertility rates declined in developing countries during the preceding 30 years, following a 20th century trend among developed countries. The pattern established by developed countries - and presumed to follow in developing countries - was that countries shift from high fertility and high mortality rates to low fertility and delayed mortality.
This transition starts with declining infant and childhood mortality as a result of improved public health measures. Improvements in infant and childhood mortality contribute to longer life expectancy and a younger population.
This trend of adults living longer, healthier lives is typically followed by a decline in fertility rates. The CDC report suggested that by 2030, there would be similar proportions of younger and older people in developing countries, by that point mirroring the age distribution in developed countries circa 1990.
Prof. Raftery's research, however, notes that in Nigeria - Africa's most populous country - each woman has an average of six children, and in the last 5 years, the child mortality rate has fallen from 136 per 1,000 live births to 117. This works out as a population increase of 20 people per square mile over the same timespan.
How will population growth affect developing countries?
But what does this mean for countries where the public health system is already stretched to breaking point - as has been demonstrated by the recent Ebola epidemic?
"Rapid population growth is likely to increase the burden on the public health service proportionally," answered Prof. Raftery.
"There are already big public health needs and challenges in high-fertility countries, and rapid population growth will make it even harder to meet them." However, if the fertility rate declines faster, Prof. Raftery suggests that high-fertility countries can reap "a demographic dividend."
"This is a period of about a generation during which the number of dependents (children and old people) is small. This frees up resources for public health, education, infrastructure and environmental protection, and can make it easier for the economy to grow. This can happen even while the population is still increasing."
Does this suggest that an increasing population is not quite as much of a threat, but that it is more specifically the accelerations and decelerations in fertility rates that provide warning signs to future public health crises?
"Following a long run of an increasing human population growth rate, over the past half century the rate has been halved from about 2% to about 1%," Darryl Holman, professor of biological anthropology at the University of Washington, explained to MNT.
More people means greater efforts are needed to control waste management and provide clean water, otherwise diarrheal disease will have a "huge" impact on mortality.
"The turnaround is quite remarkable," he said. "But as long as the growth rate remains positive, our species will eventually reach numbers and densities where technological solutions cannot ameliorate resource scarcity."
High population density leads to a much higher rate of contact between humans, which means that communicable diseases - ranging from the common cold to Dengue fever - can be much more easily transmitted.
And more people means greater efforts are needed to control waste management and provide clean water. If these needs cannot be adequately met, then diarrheal diseases become much more common, resulting in what Prof. Holman described to the University of Washington's news website The Daily UW as a "huge, huge, huge difference in mortality rates."
Taking a more general view, "the anticipated increase in the number of older persons will have dramatic consequences for public health, the health care financing and delivery systems, informal caregiving, and pension systems," wrote the authors of the CDC's 2003 report.
Overpopulation and the environment
"Can we assume that life on earth as we know it can continue no matter what the environmental conditions?," asked the authors of a 2001 Johns Hopkins School of Public Health report on the health consequences of population growth.
The Johns Hopkins report quoted figures demonstrating that unclean water and poor sanitation kill over 12 million people every year, while air pollution kills 3 million. In 64 of 105 developing countries, population has grown faster than food supplies.
By 2025, the report claimed, humankind could be using over 90% of all available freshwater, leaving just 10% for the world's plants and animals.
Prof. Holman summarizes the writings of experts Joel Cohen, E.O. Wilson, Paul Ehrlich and Ronald Lee, who have argued that the consequences of long-term environmental degradation - "specifically rising sea levels, disruption of agriculture and the increased frequency of extreme weather events resulting from anthropogenic climate change, exacerbated by resource scarcity" - create social problems that lead to social unrest.
With more people living together than ever before, it seems inevitable that this compounded social unrest would lead to increased warfare and fighting for resources.
According to the Johns Hopkins researchers, about half of the world's population currently occupies a coastal strip 200 kilometers wide - which means that 50% of us are squeezed together on just 10% of the world's land surface.
The projected flooding of these coastal regions as a result of global warming and rising sea levels could displace millions of people, result in widespread droughts and disrupt agriculture.
The Johns Hopkins team identified two main courses of action to divert these potential disasters.
An educated female workforce has financial independence and more control over reproduction. This pattern continuing across generations is associated with a decline in fertility rates.
Firstly - sustainable development. The report authors argued this should include:
- More efficient use of energy
- Managing cities better
- Phasing out subsidies that encourage waste
- Managing water resources and protecting freshwater sources
- Harvesting forest products rather than destroying forests
- Preserving arable land and increasing food production
- Managing coastal zones and ocean fisheries
- Protecting biodiversity hotspots.
The second vital area of action is the stabilization of population through good-quality family planning, which "would buy time to protect natural resources."
How to reduce fertility in a morally acceptable way?
Commenting on Prof. Raftery's finding that we may be welcoming an additional 5 billion individuals onto the planet by 2100 than had previously been estimated - a potential global population of 12.3 billion people - Prof. Holman admits that "it is difficult to know what the public health effects will be."
"By then, we may see severe petroleum and fresh water resource shortages, climate changes that affect agriculture patterns that, in turn, affect food supplies. Reducing fertility in socially and morally acceptable ways seems like one public health strategy to avoid - or at least postpone - testing some of these limits."
In Utopia, a sinister governmental organization proposes to sterilize a large percentage of the population by rolling out a secretly modified vaccine in response to a manufactured flu pandemic. Obviously, that is not a socially or morally acceptable strategy for reducing fertility - but what is?
Experts consider boosting the education of girls in developing countries to be a prime solution.
As well as acquiring more control over their reproductive life, an educated female workforce should have more opportunities of employment and of earning a living wage. Studies report that the children of educated women also have better chances of survival and will become educated themselves. This pattern continuing across generations is associated with a decline in fertility rates.
A 2011 article by the Earth Policy Institute (EPI), analyzing data from the United Nations (UN), states that "countries in which more children are enrolled in school - even at the primary level - tend to have strikingly lower fertility rates."
"Female education is especially important. Research consistently shows that women who are empowered through education tend to have fewer children and have them later. If and when they do become mothers, they tend to be healthier and raise healthier children, who then also stay in school longer. They earn more money with which to support their families, and contribute more to their communities' economic growth. Indeed, educating girls can transform whole communities."
The relationship between education, fertility and national poverty is a direct one. As the EPI authors add: "When mortality rates decline quickly but fertility rates fail to follow, countries can find it harder to reduce poverty."
The UN's 2012 Revision of the world population prospects report suggested if we make rapid reductions in family size, then it may still be possible to constrain the global population to 8 billion by 2045.
No projections are set in stone - all are contingent on what extent fertility rates will sway over the next century. And, as Prof. Holman pointed out to us, the nature of the threat posed by overpopulation has "been vigorously debated for over 200 years" with experts still not in complete accord.
For instance, in the 1980s, said Prof. Holman, the economist Julian Simon and ecologist Paul Ehrlich went on tour together, with a series of debates about the consequences of population growth.
"Ehrlich argued that continued population growth would lead to disaster for humans. Simon argued that population growth provided more people to invent new solutions to the problems confronting humans," said Prof. Holman, adding:
"Given the trends to this point, Simon has been 'more right.' One simple measure of this is mortality rates, which have decreased for most human groups. The flaw in Simon's argument may well be that we have never hit the limits of our finite earth. Positive population growth guarantees that we will, someday, hit some hard limits."
"So that," Prof. Holman concluded, "is the long term."